Significance Statement

There is growing interest in advancing the development of luminescent carbon quantum dots which has promising optoelectrical performance and excellent biocompatibility in comparison with inorganic quantum dots, and which have increased applications in bioimaging, photo-catalysis, solar energy harvesting, and many other areas.

In the recent past, approaches such as top-down and bottom-up have been used for the formation of carbon quantum dots, with the bottom-up approach being preferred in view of physiological and environmental compatibility of its prepared carbon quantum dots.

It has emerged that the use of the household microwave oven facilitates successful formation of carbon quantum dots that are water soluble and with elevated photoluminescence quantum yield.

Glucose has been widely used in the microwave-assisted hydrothermal reaction to produce deep ultraviolet carbon quantum dots at a quantum yield of up to 11%, and with an addition of polyethylene glycol there is chemiluminescence due to the alkaline condition. Apart from glucose, other carbohydrates have been used with a quantum yield of up to 9.5%. Also, poly-condensation reactions in microwave treatment using polyamidation monomer sets of A3 and B2, have been successfully used in forming carbon quantum dots with increased fluorescence quantum yield of up to 14%.

Yujin Choi and colleagues at Korea National University of Transportation used lysine in the microwave assisted preparation of carbon quantum dots, which does not freeze during poly-condensation and also the feeding ratio does not require optimization due to its single AB2 molecule structure, which is not the case with the A3 and B2 monomer types. Their work is now published in Journal of Industrial and Engineering Chemistry.

When the authors evaluated the fluorescence spectrum they showed that there was an increase in peak emission wavelength from 406nm which was purple to 497nm which was green, at 320nm and 420nm excitation wavelengths respectively. Maximum fluorescence is observed at 432nm which is blue at an excitation wavelength of 360nm. An analysis of decay due to fluorescence reveals that the fluorescence emission profile and the lateral dimensions of the lysine based carbon quantum dots are the same.

Extensive microanalysis in the study showed that lysine-based carbon quantum dots are made up of amorphous like carbon lattice, and an x-ray analysis confirms lysine-based carbon quantum dots have carbon dot-like structure and not graphene quantum dot-like structure.

Successful formation of carbon quantum dots using a household microwave with increased mass and quantum yield, confirmed the effectiveness of lysine as an amino acid-based building block in the formation of carbon quantum dot-related nanomaterials.